[M-GI29] Data assimilation: A fundamental approach in geosciences

[MGI29-11] Feasibility study of the global MHD simulation code toward reanalysis of the space weather phenomena

*Shigeru Fujita¹, Shin ya Nakano², Akira Kadokura³, Yoshimasa Tanaka³, Ryuho Kataoka³, Aoi Nakamizo⁴, Yasubumi Kubota⁴, Keisuke Hosokawa⁵, Satoko Saita⁶ (1.Meteorological College, Japan Meteorological Agency/ National Institute of Polar Research, 2.The Institute of Statistical Mathematics, 3.National Institute of Polar Research, 4.National Institute of Information and Communications Technology, 5.The University of Electro-Communications, 6.National Institute of Technology, Kitakyushu College)

Keywords:magnetosphere-ionosphere, global MHD simulation, data assimilation

We hardly obtain an accurate 3-D image of the magnetospheric phenomena only from the observations because the direct (in-situ) observations in the magnetosphere are sparse. Numerical simulations that accurately solve the physical first-principles are powerful tools for studying phenomena occurring in the magnetosphere. So, the magnetosphere-ionosphere global simulation is a useful tool for magnetospheric physics. For this purpose, we need to confirm the accuracy of the simulation results. Our final target is to present the reanalysis data in the magnetosphere-ionosphere system from the observation data and the simulation with the data assimilation technique.



There are a few global MHD simulation codes solving MHD equations in the magnetosphere-ionosphere coupled system in the world. Among them, the REPPU (REProduce Plasma universe) code developed by Tanaka [Tanaka, 2015] has a prominent property; it can reproduce the ionospheric features of the magnetospheric phenomena better than other MHD codes. Since the ionospheric and ground-based observations are far-more globally arranged compared with the in-situ magnetospheric observations, the REPPU code is suitable to reproduce the appropriate grid-point data of the magnetosphere-ionosphere region by confirming the simulation results in the ionosphere with the observations. However, the REPPU code uses free parameters to determine the ionospheric conductivities, which can be determined from assimilation between the simulation and the observation. Thus, we try to determine the free parameters from the data assimilation.



Before applying the assimilation technique to the REPPU code, we need to improve the present REPPU code. That is to say, the code uses limited information of the IMF (only By and Bz) and solar wind velocity (only Vx) because the original REPPU has been used primarily for investigating fundamental physical processes of the magnetospheric phenomena. The improved REPPU code is also made to include the effect of the magnetic axis’s seasonal tilt on the ecliptic plane. The magnetic axis’s rotation around the Earth's rotation axis is also considered in the improved REPPU. NICT first made this improvement for the operational space weather forecasts, and we made the improved REPPU code independently of NICT to obtain the code used for the assimilation study.



We started to compare the observed data with the simulation results based on the ionospheric conductivity distribution determined by the empirical parameters. The main results are as follows;
The correlation coefficient between the calculated electric potential in the polar region and the superDARN potential is about 0.8. The simulation almost reproduces the field-aligned current pattern observed by the AMPERE. However, the field-aligned current intensity and location are not wholly the same between the simulation and the observation. Therefore, the correlation between the simulation and the observation becomes low.
These results suggest that the improved REPPU code product can be regarded as primitive reanalysis data.



We are now analyzing the superMAG data to investigate how the simulation reproduces the ground magnetic variations.



References

Tanaka, T. (2015), Substorm auroral dynamics reproduced by the advanced global M-I coupling simulation, In Auroral dynamics and space weather, Geophys. Monogr. Ser., vol. 215, edited by Y. Zhang and L. J. Paxton, p. 177, doi: 10.1002/9781118978719, AGU, Washington D. C.